Ethylene selectivity control

Because of the extreme conditions in conventional direct electrofluorination, porous carbon was used in most cases, occasionally promoted with nickel (346-349). The products of such fluorinations are quite interesting for industrial processing ethylene yields vinyl fluoride and polyfluoroethanes, while acetylene forms difluoroethylene (343) acetone and alcohols give perfluorinated compounds on nickel (349). Unfortunately these studies have focused only on screening possible reactions, and thus little fundamental understanding exists for surface reactions and selectivity control. 

Hydrogenation of acetylenic to ethylenic is a classical example of selectivity control using an additive which can realise a competitive chemisorption. 

Cross Metathesis. Of the three major types of olefin metathesis, cross metathesis (CM) has been the most challenging to selectively control. CM is an intermolecular reaction between two olefins that releases ethylene gas, among a statistical mixture of combinatorial products it is a thermodynamically controlled reaction, where impurities arising from homodimerization are common. In addition, unlike ROMP, which relieves ring strain, and RCM, which forms stable 5- and 6-membered rings, CM has no enthalpic driving force (71). 

The chlorohydrin process has largely been replaced by the direct oxidation of ethylene with silver as catalyst. By-products are CO2 and water, formed by total oxidation of ethylene or EO. The reactor design is dominated by the demand for an exact temperature and selectivity control. The conversion per pass is low (about 10%) to avoid the consecutive oxidation of EO, and the unconverted ethylene is recycled. A multi-tubular reactor guarantees efficient heat transfer. 

Direct Chlorination of Ethylene. Direct chlorination of ethylene is generally conducted in Hquid EDC in a bubble column reactor. Ethylene and chlorine dissolve in the Hquid phase and combine in a homogeneous catalytic reaction to form EDC. Under typical process conditions, the reaction rate is controlled by mass transfer, with absorption of ethylene as the limiting factor (77). Ferric chloride is a highly selective and efficient catalyst for this reaction, and is widely used commercially (78). Ferric chloride and sodium chloride [7647-14-5] mixtures have also been utilized for the catalyst (79), as have tetrachloroferrate compounds, eg, ammonium tetrachloroferrate [24411-12-9] NH FeCl (80). The reaction most likely proceeds through an electrophilic addition mechanism, in which the catalyst first polarizes chlorine, as shown in equation 5. The polarized chlorine molecule then acts as an electrophilic reagent to attack the double bond of ethylene, thereby faciHtating chlorine addition (eq. 6) ... 

A selective poison is one that binds to the catalyst surface in such a way that it blocks the catalytic sites for one kind of reaction but not those for another. Selective poisons are used to control the selectivity of a catalyst. For example, nickel catalysts supported on alumina are used for selective removal of acetjiene impurities in olefin streams (58). The catalyst is treated with a continuous feed stream containing sulfur to poison it to an exacdy controlled degree that does not affect the activity for conversion of acetylene to ethylene but does poison the activity for ethylene hydrogenation to ethane. Thus the acetylene is removed and the valuable olefin is not converted. 

A carbonyl group cannot be protected as its ethylene ketal during the Birch reduction of an aromatic phenolic ether if one desires to regenerate the ketone and to retain the 1,4-dihydroaromatic system, since an enol ether is hydrolyzed by acid more rapidly than is an ethylene ketal. 1,4-Dihydro-estrone 3-methyl ether is usually prepared by the Birch reduction of estradiol 3-methyl ether followed by Oppenauer oxidation to reform the C-17 carbonyl function. However, the C-17 carbonyl group may be protected as its diethyl ketal and, following a Birch reduction of the A-ring, this ketal function may be hydrolyzed in preference to the 3-enol ether, provided carefully controlled conditions are employed. Conditions for such a selective hydrolysis are illustrated in Procedure 4. 

Phosphoric acid esters based on alkylene oxide adducts are of great interest. Their properties can be altered by the length and structure of the hydrophobic alkyl chain. But they are also controlled by the kind and length of the hydrophilic alkyleneoxide chain. The latter can easily be tailored by selection between ethylene oxide and propylene oxide and by the degree of alkoxylation. 

---